Commentaire officiel
The OBJ, MTL and JPG files can be uploaded directly to give the best resolution possible. The KMZ is a 250K face 3D model. Depending on the kind of SketchFab account you have you can upload a few million faces and have better 3D fidelity.
The Salvation Mountain sample was directly uploaded as the OBJ file set.
If it is giving you a warning that the files are too big you can use MeshLab (free software) to decimate the model down to a size that will be capable of being uploaded to your account.